Java quick reference

Program skeleton

import java.util.Scanner;

public class Main {                          // file: Main.java
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        System.out.println("n = " + n);
    }
}

Compile javac Main.java · run java Main. JDK ⊃ JRE ⊃ JVM.

Primitive types

Widening (automatic): byte → short → int → long → float → double; char → int. Narrowing needs a cast: (int) 3.9 → 3.

Operator precedence (high → low)

Control flow

if (a > b) { ... } else if (a == b) { ... } else { ... }

switch (day) { case 1: ...; break; default: ... }
String t = switch (day) { case 6, 7 -> "Weekend"; default -> "Weekday"; };

for (int i = 0; i < n; i++) { ... }
for (int x : arr) { ... }
while (cond) { ... }
do { ... } while (cond);

outer: for (...) { for (...) { if (done) break outer; if (skip) continue outer; } }

Arrays

int[] a = new int[5];          int[] b = {3, 1, 2};
int[][] m = new int[3][4];     int[][] j = { {1}, {2, 3} };
a.length   m[0].length
Arrays.sort(b);  Arrays.toString(b);  Arrays.copyOf(b, 5);
Arrays.fill(a, 7);  Arrays.equals(a, b);  Arrays.binarySearch(b, 2);

Classes, objects and constructors

public class Student {
    private String name;                  // field
    private static int count = 0;         // class-wide
    public Student() { this("Unknown"); } // constructor chaining
    public Student(String name) { this.name = name; count++; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    @Override public String toString() { return "Student " + name; }
}
Student s = new Student("Ann");

• No constructor written → Java adds a default no-arg one.
• Fields default to 0 / 0.0 / false / null; local variables have no default.

Access modifiers

static and final

Overloading vs overriding

Inheritance and polymorphism

class Dog extends Animal {
    Dog(String n) { super(n); }                 // first statement
    @Override String sound() { return "Woof"; }
    String both() { return super.sound() + sound(); }
}
Animal a = new Dog("Rex");        // upcast (automatic)
a.sound();                        // Dog's version runs
if (a instanceof Dog d) d.fetch(); // pattern + downcast

Constructors run top-down (Object → Animal → Dog). A class extends one class and implements many interfaces.

Abstract classes and interfaces

interface Payable { double getPayment(); default String currency() { return "BDT"; } }
abstract class Shape { abstract double area(); }

Packages

package bd.edu.green.cse.model;       // first line of the file
import java.util.ArrayList;           // one class
import java.util.*;                   // whole package (not sub-packages)
import static java.lang.Math.PI;      // static import

java.lang is imported automatically. Folder path = package name.

Exceptions

try {
    risky();
} catch (FileNotFoundException e) {     // specific first
    ...
} catch (IOException | NumberFormatException e) {
    System.out.println(e.getMessage());
} finally {
    cleanUp();                           // always runs
}
void read() throws IOException { throw new IOException("disk"); }
class MyException extends Exception { MyException(String m) { super(m); } }
try (Scanner sc = new Scanner(new File("a.txt"))) { ... }   // auto-close

Checked: Exception but not RuntimeException (IOException, SQLException, InterruptedException). Unchecked: RuntimeException (ArithmeticException, NullPointerException, ArrayIndexOutOfBoundsException, NumberFormatException, ClassCastException).

Threads

Thread t = new Thread(() -> work(), "worker");
t.start();  t.join();  Thread.sleep(500);
class MyThread extends Thread { public void run() { ... } }

synchronized void inc() { count++; }
synchronized (lock) { ... }
while (!ready) wait();   // inside synchronized
notifyAll();

States: NEW → RUNNABLE ⇄ BLOCKED / WAITING / TIMED_WAITING → TERMINATED.

String methods

StringBuilder sb = new StringBuilder();
sb.append("a").append(1).insert(0, "[").reverse().toString();
BigInteger big = new BigInteger("123456789012345678901234567890").multiply(BigInteger.TEN);

JavaFX

public class App extends Application {
    public void start(Stage stage) {
        Button b = new Button("OK");
        b.setOnAction(e -> System.out.println("clicked"));
        VBox root = new VBox(10, new Label("Hi"), b);
        stage.setScene(new Scene(root, 300, 200));
        stage.show();
    }
    public static void main(String[] a) { launch(a); }
}

Layouts: HBox, VBox, FlowPane, GridPane (add(node, col, row)), BorderPane (setTop/Center/…), StackPane. Drawing: Canvas → getGraphicsContext2D() → fillRect, fillOval, strokeLine, fillText. Animation: AnimationTimer.handle(now).

JDBC

try (Connection con = DriverManager.getConnection(url, user, pass);
     PreparedStatement ps = con.prepareStatement("SELECT name FROM students WHERE cgpa >= ?")) {
    ps.setDouble(1, 3.5);
    try (ResultSet rs = ps.executeQuery()) {
        while (rs.next()) System.out.println(rs.getString("name"));
    }
}                              // executeUpdate() for INSERT / UPDATE / DELETE